Dual video streaming 1080p+720p works okay on iMX6Q but not on iMX6QP

I use an ov5640 camera module for H.264 network streaming on two boards (nitrogen6_max): one with iMX6QP the other "only" with iMX6Q with a current Yocto morty image (Linux nitrogen6x 4.1.15-2.0.0-ga+yocto+gfc4f6c4, GStreamer 1.8.3).

Now, I tried to repeat this setup on my iMX6QP board in hope for a reduced bus load and, maybe, enough headroom for a third (tiny preview) stream. But that failed. The bus load is a little lighter, as expected:

stress test runs fine on my board. I doubt that a memory error would show the kind of artefacts I see. It's not corrupted data but missing/skipped data. Dual 1080p streaming also runs fine. Could it be an issue with AXI priorities for IPU1 and IPU2?